1. Field of Invention
This invention pertains to porous material fabrication by controlling pore size, pore distribution, pore volume fraction and thus the mean nearest-neighbor distance between the pores using beads which are thermally decomposable to a gas.
2. Description of Prior Art
The principal class of porous material products, particularly porous metal products, are made by powder metallurgy. The powder metallurgy process includes primarily the steps of mixing batch components, including particulate or powdered material; compacting the batch components to form a green body which holds its shape; and sintering the green body at elevated temperature and pressure to form a porous material. The porous material is typically machined to obtain a final porous material product.
Casting involves melting and molding of hot liquid material into product and cooling the molten material to a solid state. One form of casting is the lost-foam casting process wherein connected pores in a plastic foam are filled with a refractory which is then cured. Upon heating combustion of the sacrificial foam leads to formations of a sponge-like solid. This solid is typically used as a mold for material which solidifies in its pores. This process is primarily used for making porous metals with low melting points.
Melt foam generation is hard to control and foamed materials contain large bubbles non-uniformly distributed throughout the casting. Numerous attempts have been made to prevent this defect, as by vigorous stirring, application of magnetic fields, or thickening. Problems, however, persist due to the relatively short time between the introduction of the foaming agent and the generation of foam. Further difficulties arise from premature decomposition of the hydride or another foaming agent. Thickening additives often impair mechanical properties of the foamed material.
Infiltration of a granular bed is in many respects similar to lost-foam casting. It yields a continuous structure by melt infiltration of a bed of granules contained in the casting mold. The granules are made of a soluble but thermally stable material that is removed by chemical treatment. To ensure free flow of the melt, it should be superheated. It is also desirable to preheat the granule bed and pressurize the melt or evacuate the spaces between the granules. An alternative method involves introducing granules into the melt while it is vigorously stirred.
A rather recent fabrication of foamed aluminum involves introduction of air or another gas into a molten aluminum puddle while simultaneously stirring the melt in the bubbling zone. A surfactant is concurrently injected with the gas to stabilize the foam. In this continuous process, the foam is fed into a horizontal or vertical mold to freeze, forming sheets, tubes, or other products with porous structure. The method offers high output, is simple and cost-efficient, and allows pore-size control over a fairly wide range. An important advantage of this technology is the possibility of using aluminum of any degree of purity, and thus recycling aluminum scrap.
In the gas-eutectic transformations of the metal-hydrogen systems, the liquid decomposes into a solid and a gas phase: L.fwdarw.S+G. The transformation may take place if the phase diagram for the metal-hydrogen system involves a gas-eutectic equilibrium. Making the material includes two steps:
a. charging a molten material with hydrogen to reach the eutectic composition, and PA1 b. solidification in a conventional or continuous casting mold.
No melt foaming occurs here because the gas is evolved as the melt freezes. The process is in many ways similar to conventional eutectic solidification, the distinction being that the liquid decomposes into a solid and a gas rather than into two solids. The main process variables that govern the amount of porosity and the size, shape, and orientation of the pores are the hydrogen level in the melt, gas pressure over the melt during solidification, direction and rate of heat removal, and the material chemical composition. By changing these variables, one can control the pore structure over a wide range.